Functional heterogeneity among macrophages (MO) and T cells is well documented. To investigate the possibility that among human MO there existed an antigen presenting subpopulation, monoclonal antibodies were utilized. Using both cytolytic depletion and positive selection techniques it was demonstrated that a MO subpopulation (about 40%) depicted by the monoconal antibody, Mac-120, was crucially required for the genetically restricted presentation of antigen to reactive T cells. Moreover, only Mac-120 MO are able to stimulate T cells in the autologous MLR. Therefore Mac-120+ MO are required to activate T cells in the presence or absence of exogenous antigen. Both MO populations bear quantitatively equivalent concentrations of HLA-DR antigens and liberate similar quantities of IL-1. Thus, the only observable difference between these populations is the presence on the Mac-120+ MO of the determinant recognized by the antibody. To determine whether this molecule is a cell interaction determinant reactive with a specific T cell receptor, the following hypothesis was proposed. If the Mac-120 antibody and a T cell receptor both interact with the same MO determinant, then they may share molologous peptide sequences. Thus if an anti-idiotypic antibody to the Mac-120 antibody were produced, it might also possess specificity to the T cell receptor. To test this hypothesis rabbits were immunized with Mac-120, the immune (IS) serum was absorbed with mouse Ig and assayed against human T cells. Although the anti-idiotypic nature of the serum has not been definitively proven, several experiements are of note. In immunofluorescence and cytotoxicity studies, with normal rabbit serum as the control, the IS reacts with 5-15% of peripheral blood T cells. When added to mitogen, antigen or autologous MO induced T cell proliferation cultures, a dramatic diminution of reactivity is apparent. These data suggest that the T cell population reactive with IS is required for full expression of T cell proliferation. It is the purpose of this proposal to accomplish the following: First, to demonstrate that indeed IS contains anti-idiotypic antibody to Mac-120. Second, to generate a monoclonal antibody to the T cell receptor, such that cloning and functional characterization are possible. Third, to study interactions among Mac-120+ and Mac-120-MO and IS+ and IS-T cells which culminate in the generation of effector or suppressor T cells. And finally to use the cloned T cells and monoclonal antibody to isolate the T cell receptor and by 2-D gel electrophoresis to compare it to the Mac-120 H chain.